The present invention relates to an image forming device and an image forming method. More particularly, the present invention relates to an image forming device and an image forming method by which image data are transferred to a recording engine after the image data having been compressed and decompressed.
In laser printers, the laser beam modulated according to the image data is made to irradiate a polygon mirror that is driven to rotate at constant rate and the laser beam reflected from the polygon mirror is made to raster-scan a photosensitive body to form an electrostatic latent image on the photosensitive body. Therefore, it is necessary to supply image data that correspond to a raster to the laser scanner unit (to be referred to simply as “laser unit” hereinafter) disposed in the laser engine, that is a recording engine of the laser printer, in synchronism with the rotary motion of the polygon mirror that is rotated at a rate determined as a function of the printing speed (the number of sheets printed in a unit time) and the resolution (the number of dots printed on a unit length) within a predetermined period of time (printing time for a raster).
In recent years, the amount of data that needs to be supplied to the laser unit within a unit time has been remarkably increased for the purpose of realizing color printing and high speed printing. However, it is not possible to transfer the image data stored in the RAM (memory) to some other part at a sufficiently high rate because of the restrictions imposed on the bus width and the bus cycle of the RAM arranged on the main substrate of the laser printer, which is adapted to receive printing data (original data) from the host computer, convert them into image data and subsequently supply them to the laser unit. Therefore, unless the RAM is replaced by some other one that affords a greater bus width and a shorter bus cycle and hence is more costly, it is no longer possible to supply image data by the amount required for the laser unit from the main substrate to the laser unit within the time for printing a raster. Then, the laser printer can become overloaded. To avoid this problem, there has been proposed a technique with which the image data are compressed before they are stored in the RAM and transferred to some other part (e.g., ASIC (application specific integrated circuit) and subsequently the compressed image data are decompressed (expanded) so that the decompressed image data are supplied to the laser unit.
However, the amount of data that can be reduced by compression can vary depending on the entropy of the data. Therefore, when a relatively high constant speed is defined for printing, assuming that a good and high compression ratio can be put to use, there can arise a situation where it is no longer possible to supply data by the required amount to the laser unit within the period of time for printing a raster unless a costly RAM that affords a greater bus width and a shorter bus cycle is employed if the image data to be used for printing has not been compressed to a high compression ratio. Then, the laser printer can become overloaded, i.e., print overrun occurs. On the other hand, when a relatively low constant speed is set for printing, assuming that only a poor and low compression ratio can be put to use, there can arise a situation where a low printing speed is in place and the printing engine is not exploited to perform highly, although image data can be compressed to a high compression ratio to realize a high speed printing operation.
Japanese Patent Application Laid-Open Publication No. 6-316118 describes a technique for performing a efficient printing when the data of a page (the intermediate codes generated from the printing data) are divided into a plurality of bands and the data are processed on a band by band basis in a laser printer. According to the technique, the printing speed is lowered when the maximum amount of information of a band is too large, since a prolonged time period is required for developing data from a display list memory storing the divided intermediate codes into a page data memory storing the intermediate codes of one page. On the other hand, the printing speed is raised when the maximum amount of information of a band is not very large, because developing data from the display list memory to the page data memory is not time consuming. The disclosed technique is designed to regulate the printing speed according to the time period required for developing the intermediate codes that are divided into a plurality of bands into data of one page in a step preceding the step of compressing image data. The disclosed technique does not concern the compressed image data transmission efficiency from a RAM, the image data having been converted from the intermediate codes and have been compressed.